Candida Invasion of Endothelium and Virulence Abstract The incidence of bloodstream infections caused by Candida species has increased dramatically; these organisms now account for 10% of all bloodstream isolates. The mortality associated with candidemia approaches 40%, even with therapy. Because of this unacceptably high mortality, new strategies to treat and prevent this infection are necessary. Our goal is to identify new Candida albicans virulence genes and to determine the mechanisms by which they contribute to pathogenicity. This information holds promise to identify new targets for antifungal strategies. Our hypothesis is that the ability of C. albicans to invade and damage host cells, while avoiding host defense mechanisms, is critical for the organism to establish and maintain a deep- seated infection. Support for this hypothesis comes from our finding that many C. albicans mutants with reduced capacity to invade and damage endothelial cells in vitro also have decreased virulence in the mouse model of hematogenously disseminated infection. Our objective is to use in vitro studies of the interactions between C. albicans and vascular endothelial cells to define mechanisms of host- pathogen interaction. C. albicans invades endothelial cells by inducing its own endocytosis. In the previous project period, we discovered that C. albicans Als3 is an invasin that binds to N-cadherin on the endothelial cell surface and induces the endocytosis of the organism. Recently, we have determined that there are additional C. albicans invasins and endothelial cell surface proteins that mediate endocytosis. These surface proteins include AHNAK and ErbB2. In this project, we will 1) determine the interactions among AHNAK, N-cadherin, and ErbB2 in endothelial cell invasion by C. albicans; 2) determine the mechanisms by which C. albicans Ssa1 contributes to endothelial cell invasion and virulence; 3) use C. albicans mutants to elucidate functional relationships among C. albicans virulence regulators, endothelial cell receptors, and endothelial cell damage.